An electroluminescent (EL) device is a self-light-emitting device with the advantages of providing a wider viewing angle, a greater contrast ratio, and a faster response time. The first organic EL device was developed by Eastman Kodak, by using small aromatic diamine molecules and aluminum complexes as materials for forming a light-emitting layer (see Appl. Phys. Lett. 51, 913, 1987).
An organic EL device changes electric energy into light by the injection of a charge into an organic light-emitting material, and commonly comprises an anode, a cathode, and an organic layer formed between the two electrodes. The organic layer of the organic EL device may be composed of a hole injection layer, a hole transport layer, an electron blocking layer, a light-emitting layer (containing host and dopant materials), an electron buffer layer, a hole blocking layer, an electron transport layer, an electron injection layer, etc.; the materials used in the organic layer can be classified into a hole injection material, a hole transport material, an electron blocking material, a light-emitting material, an electron buffer material, a hole blocking material, an electron transport material, an electron injection material, etc., depending on functions. In the organic EL device, holes from an anode and electrons from a cathode are injected into a light-emitting layer by electric voltage, and an exciton having high energy is produced by the recombination of the holes and electrons. The organic light-emitting compound moves into an excited state by the energy and emits light from energy when the organic light-emitting compound returns to the ground state from the excited state.
The most important factor determining luminous efficiency in an organic EL device is light-emitting materials. The light-emitting materials are required to have the following features: high quantum efficiency, high movement degree of an electron and a hole, and uniformity and stability of the formed light-emitting material layer. The light-emitting material is classified into blue, green, and red light-emitting materials according to the light-emitting color, and further includes yellow or orange light-emitting materials. Furthermore, the light-emitting material is classified into a host material and a dopant material in a functional aspect. Recently, an urgent task is the development of an organic EL device having high efficiency and/or long lifespan. In particular, the development of highly excellent light-emitting material over conventional materials is urgently required, considering the EL properties necessary for medium- and large-sized OLED panels. For this, preferably, as a solvent in a solid state and an energy transmitter, a host material should have high purity and a suitable molecular weight in order to be deposited under vacuum. Furthermore, a host material is required to have high glass transition temperature and pyrolysis temperature for guaranteeing thermal stability, high electrochemical stability for long lifespan, easy formability of an amorphous thin film, good adhesion with adjacent layers, and no movement between layers.
Further, the electron buffer layer is equipped to improve a problem of light-emitting luminance reduction which may occur due to the change of current properties in the device when the device is exposed to a high temperature during a process of producing panels. Thus, the properties of the compounds comprised in the electron buffer layer are important. In addition, the compound used for the electron buffer layer performs a role of controlling an electron injection by the electron withdrawing characteristics and the electron affinity LUMO (lowest unoccupied molecular orbital) energy level, and thus performs a role to improve the efficiency and lifespan of the organic electroluminescent device.
Meanwhile, in an organic EL device, an electron transport material actively transports electrons from a cathode to a light-emitting layer and inhibits transport of holes which are not recombined in the light-emitting layer to increase recombination opportunity of holes and electrons in the light-emitting layer. Thus, electron-affinitive materials are used as an electron transport material. Organic metal complexes having light-emitting function such as Alq3 are excellent in transporting electrons, and thus have been conventionally used as an electron transport material. However, Alq3 has problems in that it moves to other layers and shows reduction of color purity when used in blue light-emitting devices. Therefore, new electron transport materials have been required, which do not have the above problems, are highly electron-affinitive, and quickly transport electrons in organic EL devices to provide organic EL devices having high luminous efficiency.
Korean Patent No. 1297158 discloses a compound wherein a heteroaryl is bonded to a carbon position other than a naphthalene ring in a naphthoxazole structure via an arylene as a linker; Korean Patent Appln. Laying-Open No. KR 2008-0028424 A discloses a naphthoimidazole derivative; European Patent Application Publication No. EP 1593675 A1 discloses a compound wherein an amine is bonded to a carbon position of a benzene ring, which is directly fused with an oxazole in a naphthoxazole structure, via arylene as a linker; and Korean Patent No. 1202349 discloses a benzoindene derivative. However, the above references fail to disclose a compound wherein a heteroaryl is bonded to a carbon position of a benzene ring, which is not directly fused with an oxazole in a naphthoxazole structure, directly or via an arylene as a linker.
In addition, Korean Patent Appln. Laying-Open No. KR 2015-0136033 A discloses a compound wherein a heteroaryl is bonded to a carbon position of a benzene ring, which is not directly fused with an oxazole in a naphthoxazole structure, directly or via an arylene as a linker. However, the compound disclosed in KR 2015-0136033 A is different from the compound of the present disclosure in the position of the substituents.